In recent years nutritional management of the low-birth-weight infant has become a major concern in neonatal centers throughout the country. Coincident with increasing survival of the very small preterm baby, a renewed interest in breast feeding of even the very high risk infant has occurred. Unfortunately, no data are available regarding the composition of the breast milk of a premature infant's mother, and only scanty information can be found describing the composition of mature breast milk. The present investigation is designed to provide comprehensive, reliable data of the composition of human breast milk as well as the adequacy of human milk for feeding of the premature infant. The proposed study is divided into two major areas of investigation. The first year will be devoted to the analysis of the constituents of human milk in relation to gestational age and duration of lactation. The second and third years of study are intended to apply knowledge gained in the first year to a clinical prospective trial of breast milk feeding of the preterm baby. By carefully controlling the study population and monitoring a variety of clinical and biochemical parameters, it can be determined whether human milk feeding of the preterm infant is an adequate, inadequate or superior source of nutrition than standard cow's milk formulas.